Coordinate measuring machines are well known in the prior art. One type of coordinate measuring machine has a probe which is mounted with its axis vertical and which is movable up and down to make measurements of the part. The vertically mounted probe is typically mounted to a carriage which is movable horizontally. The movement of the probe or carriage is accomplished either by an operator's grasping the probe and physically moving it in the desired direction in one type of machine or by a motor and automatic controller driving the probe in a more sophisticated type of machine.
In order to accomplish the part measurement, the probe and a carriage must be easily movable. Ideally, the probe and carriage and related apparatus must be light weight.
Further, the coordinate measuring machine must be very accurate. Such accuracy is dependent upon a low friction, which in turn is related to the weight of the movable apparatus which is supported on a fixed apparatus by bearings.
Further, it is desired to have a system which is compact so that the measuring machine is no larger than necessary. As the size of the machine increases, the required length of precision components (such as bearing rails and measuring scales) increases, adding to the cost of manufacturing. Furthermore, a larger machine disadvantageously requires more space in the manufacturing environment, where space conservation is desirable.
The vertically mounted probes must be easily movable in response to the desired movement by an operator. However, the probes must not be movable freely or when no movement is desired, as otherwise the weight of such probe would present a safety hazard as the weight of the probe would bring it down.
Several systems have been suggested to counterbalance the weight of the probe shaft to prevent its falling downward and thereby possibly causing injury or damage. The prior art counterbalancing systems include a mechanical counterbalance, electrical counterbalance and pneumatic counterbalances.
One method of accomplishing the mechanical counterbalance is to provide the probe shaft with an equivalent weight mounted to and offsetting the weight of the probe. Such an arrangement is disadvantageous in that equivalent the weight adds significantly to the friction in the system and makes it more difficult to move the probe and the carriage to accomplish part measurement.
Another mechanical counterbalance approach in the prior art couples the probe to a spring to provide an equivalent force directed opposite to the weight of the probe. Unfortunately, springs are not available which exert a uniform force over a typical operating range of probe shaft, movement which is between 8 and 40 inches depending on the size of the machine. Furthermore, the use of a spring counterbalance could result in friction which is greater than desirable in some applications.
An electric counterbalance is disclosed in U.S. Pat. No. 3,818,596. In the patent, a counterbalance arrangement is disclosed which uses a variable torque, magnetic particle clutch which has an output which is varied in accordance with the weight of the probe shaft.
Air or penumatic counterbalances have also been suggested in the prior art. The prior art air counterbalances have typically been of large size to allow for the air pressure to act on a member which moves a distance equivalent to the vertical movement of the probe.
Also, the prior art air counterbalances have undesirably high inertia and friction.
Unfortunately, these counterbalances have undesirable friction which makes it more difficult to move the probe shaft than is desirable. Further, these systems have characteristically been large and heavy, which have an unsatisfactory effect on the accuracy and repeatability of the coordinate measurements.
According, there is a need in coordinate measuring machines for an improved counterbalancing system for the vertically mounted probe and prior art systems have significant undesirable features and limitations.